Conventional waveguide structures, which have been fabricated for signals up to around 600 GHz, comprise a discrete planar diode mounted on a microstrip of active components which in turn is 1o mounted on a separately fabricated support. Difficulties have been encountered though in scaling down such structures for higher frequency signals due to limitations encountered in mounting the diode on the microstrip and the parasitic capacitive effects resulting from the diode chip/microstrip combination.